Prenatal detection of distal 18p deletion by chromosomal microarray analysis: Three case reports and literature review

Background: Chromosome 18p deletion syndrome is caused by total or partial deletion of the short arm of chromosome 18 and associated with cognitive impairment, growth retardation and mild facial dysmorphism. However, most studies on the genotype-phenotype correlations in the 18p region are diagnosed postnatally. Prenatal reports involving 18p deletions are limited. Methods: Three pregnant women opted for invasive prenatal testing due to noninvasive prenatal testing indicating high risk for chromosome 18 abnormalities. Karyotypic analysis and chromosomal microarray analysis (CMA) were performed simultaneously. The pregnancy outcomes for all cases were followed up. Meanwhile, we also made a literature review on prenatal phenotypes of 18p deletions. Results: G-banding analysis showed that 2 fetuses presented abnormal karyotypes: 45,XN,der(18)t(18;21)(p11; q11),-21 (case 2) and 46,XN,18p- (case 3). The karyotype of case 1 was normal. Meanwhile, CMA detected 4.37 Mb (case 1), 7.26 Mb (case 2) and 14.97 Mb (case 3) deletions in chromosome 18p region. All 3 pregnancies were terminated finally according to genetic counseling based upon abnormal CMA results. Conclusion: Prenatal diagnosis of 18p deletion syndrome is full of challenges due to the phenotypic diversity, incomplete penetrance and lack of prenatal phenotypes. Increased nuchal translucency and holoprosencephaly are common prenatal phenotypes of distal 18p deletion. For fetuses carrying 18p deletions with atypical sonographic phenotypes, noninvasive prenatal testing could be adopted as an effective approach.


Introduction
Chromosome 18p deletion syndrome, first described by de Grouchy et al, is a rare chromosomal disorder caused by total or partial deletion of the short arm of chromosome 18. [1]The incidence of chromosome 18p deletion syndrome is estimated to be 1 in 50,000 newborn infants. [2]In patients with del(18p) syndrome, about two-thirds of the cases have a de novo pure terminal deletion, while one-third of the cases are caused by a de novo unbalanced translocation, malsegregation of a parental translocation or inversion, or a ring chromosome or isochromosome 18q. [3]The clinical phenotypes of chromosome 18p deletion syndrome is wide due to the diverse deleted fragments and the breakpoints.Patients with 18p deletion could present distinct phenotypes, including cognitive impairment, growth retardation, and mild facial dysmorphisms. [4]or decades, karyotyping has been regarded as the gold standard for detecting chromosome abnormalities in prenatal diagnosis.However, this technique can hardly identify chromosomal aberrations smaller than 5 Mb due to its low resolution.With the development of molecular genetic testing technology, chromosomal microarray analysis (CMA) has gradually become the main diagnostic test method for genetic evaluation due to higher resolution and rapid detection ability. [5]In recent years, some reports involving 18p deletion in a molecular level have been gradually reported worldwide.
Till now, most studies on the genotype-phenotype correlations in the 18p region were diagnosed postnatally.Prenatal reports involving 18p deletions were limited.Given the rare prenatal reports and diverse clinical manifestations of this syndrome, prenatal diagnosis for this chromosome disorder is still crucial and challenging.Herein, we present 3 cases of prenatal diagnostic 18p deletions with abnormal NIPT results and provide a systematic summary of prenatal phenotypes for such genomic disorders.

Methods
Our study protocol was approved by the Ethics Committee of the First Hospital of Jilin University (2021-706), and the written informed consents were obtained from the couples for publication of this case report and accompanying images.

Cytogenetic analysis
Chromosomal karyotypic analysis with a resolution of 300 to 400 bands was performed on G-band metaphases prepared from cultured aminotic fluid cells and peripheral blood cells according to standard protocols in our laboratory.Twenty metaphases were analyzed for all samples.The International System for Human Cytogenetic Nomenclature (ISCN 2016) nomenclature was used to describe all karyotypes. [6]

Case 1
A 33-year-old, gravida 2, para 1, pregnant woman underwent noninvasive prenatal testing (NIPT) at 21 weeks of gestation, which indicated high risk of chromosome 18 in the fetus.Subsequently, the woman underwent amniocentesis for cytogenetic analysis and CMA detection.G-banding analysis showed that the karyotype of the fetus was 46,XN (Fig. 1A).Meanwhile, CMA revealed a 4.37 Mb deletion in the region of 18p11.32p11.31.To identify the origin of this deletion, CMA detection for the couple was recommended.Unfortunately, the couple declined further genetic testing and chose to terminate the pregnancy at 23 weeks' gestation.

Case 2
A 36-year-old, gravida 1, para 0, pregnant woman underwent NIPT at 19 weeks of gestation, which high risk of chromosome 18 in the fetus.Subsequently, the woman underwent amniocentesis for cytogenetic analysis and CMA detection.G-banding analysis showed that the karyotype of the fetus was 45,XN,der(18)t(18;21)(p11;q11),-21 (Fig. 1B).Meanwhile, CMA revealed a 7.26 Mb deletion in the region of 18p11.32p11.23.Subsequently, the couple accepted karyotype analysis to determine the origin of the variant.The karyotype analysis showed that their karyotypes were both normal, which indicated that the deletion in the fetus was de novo.Finally, the couple chose to terminate the pregnancy at 21 weeks' gestation according to genetic counseling.

Case 3
A 26-year-old, gravida 1, para 0, pregnant woman underwent NIPT at 18 weeks of gestation, which indicated high risk of chromosome 18 in the fetus.Subsequently, the woman  underwent amniocentesis for cytogenetic analysis and CMA detection.G-banding analysis showed that the karyotype of the fetus was 46,XN,18p-(Fig.1C), and CMA revealed a 14.97 Mb deletion in the region of 18p11.32p11.21.Meanwhile, the couple opted for CMA detection for further verification.The results of CMA for the couple were normal, indicating that the deletion de novo.Finally, the couple chose to terminate the pregnancy at 20 weeks' gestation according to genetic counseling based upon abnormal CMA results.

Discusstion
In our study, we reported 3 prenatal cases carrying 18p deletions using CMA and karyotypic analysis.Case 1 carried a 4.37 Mb deletion in the 18p11.32p11.31region.Case 2 had a 7.26 Mb deletion in the 18p11.32p11.23 region.Case 3 had a 14.97 Mb deletion in the 18p11.32p11.21region.No abnormal ultrasound findings were observed in these cases during the pregnancy periods.
18p deletion syndrome (OMIM: #146390), an uncommon chromosomal submicroscopic imbalance, is characterized by mental retardation, growth retardation, craniofacial dysmorphism (round face, short protruding philtrum, palpebral ptosis, dysplastic ears, wide mouth and dental anomalies) and abnormalities of the limbs, genitalia, brain, and heart.Till now, more than 300 cases have been reported worldwide, but most of them are postnatal cases, and prenatal reports are particularly rare.18p deletions are usually detected through chorionic villus sampling or amniocentesis in prenatal settings.Moreover, it is notable that some clinic features of 18p deletions can hardly be detected in prenatal ultrasound examinations.
In this study, we report 3 prenatal cases carrying distal 18p deletions, spanning from 4.37 Mb to 14.97 Mb.In order to establish a better understanding on the antenatal phenotypes, we summarized the clinical manifestations of prenatal cases sharing similar 18p11.3210][11][12][13][14][15][16][17][18] All 18p11.32     According to the DECIPHER database, 5 morbid genes are located in the 18p11.32region detected in our cases (Table 2), which are correlated with diverse diseases.As known, the haploinsufficiency for genes would result in genetic disorders.In order to predict the potential genes associated with poor prognostic phenotypes, we delineated their functions and implications in different processes.
TGIF1 (OMIM:602630) encodes transforming growth factorβ-induced factor, which belongs to a family of evolutionarily conserved, atypical homeodomain proteins that act as transcriptional repressors and co-repressors in retinoid and transforming growth factor signaling pathway. [19]TGIF1 has been proposed as a candidate gene which is implicated in the etiology of HPE in patients with chromosome 18p deletions.28] TYMS (OMIM: 188350) encodes thymidylate synthase, which is essential for DNA replication and repair. [29][32] THOC1 and LPIN2 genes have been rarely studied in patients carrying 18p deletions, and their functions need to be further investigated.
As a promising technique, NIPT has been frequently applied to determine fetal sex, aneuploidies, microdeletions/microduplications in prenatal screening in recent years. [33]In our study, NIPT indicated a high risk of chromosome 18 for all pregnancies, which further prompted the pregnant women to opt for prenatal diagnosis and ultimately led to the detection of 18p deletion.
In our study, all couples chose to terminate their pregnancies based on genetic counseling.For case 2 and case 3 couples, preimplantation genetic testing was an ideal option and prenatal diagnosis is necessary if they intended to conceive again.For case 1, CMA testing was still necessary to determine whether the 18p deletion was de novo or inherited from the parents.

Conclusion
We describe 3 prenatally diagnosed cases with 18p deletion by karyotype and CMA.Fetuses with 18p microdeletions could exhibit diverse ultrasound findings, ranging from normal to abnormal.Increased NT and HPE are common prenatal phenotypes of distal 18p deletion.In addition, NIPT could be adopted as an effective approach in detecting 18p deletion in prenatal setting, especially when no abnormal ultrasound findings were observed.Our study would provide a better prenatal management for the genetic detection and clinic counseling of 18p deletions.

Table 1
The prenatal phenotypes of published literature and present cases with 18p deletion.
microdeletions were varied in size, from 3 Mb to 18 Mb.Among these deletions, 4/19 were parentally inherited, 12/19 cases were de novo, and 3/19 cases were not available.Among them, 12/19 cases presented ultrasound abnormalities.Increased nuchal translucency (NT) was observed in 4/19 cases and holoprosencephaly (HPE) was discovered in 5/19 cases.Besides, hydronephrosis was observed in 2/19 cases and cardiac abnormality was discovered in 1/19 cases.It was worth noting that the remaining 7/19 cases without ultrasound abnormalities were detected using NIPT (6/19) and maternal serum screening(1/19).The termination of pregnancy was opted for all cases except case 6.Based upon the fact mentioned above, increased NT and HPE are common prenatal phenotypes of distal 18p deletion, and NIPT plays a critical role in detecting fetuses with 18p deletions presenting no abnormal ultrasound findings.

Table 2
Overlapping genes in the 18p deletion region in our case.